The present invention relates to an image display apparatus having reduced emissions and enhanced clarity.
A common type of image display apparatus comprises an image signal processing circuit that receives and amplifies an image signal, and a cathode-ray tube with high-voltage electron guns that displays the amplified image signal. Due in part to the combination of high cathode voltages with high image-signal frequencies, such apparatus emits unintended electromagnetic radiation. To prevent interference with other electronic equipment, and for the safety of the viewer, the unintended radiation must remain within established limits. The frequency spectrum of the unintended radiation must not have peaks exceeding the established limit levels.
A resistor inserted between the signal-processing circuit and the cathode-ray tube is a common means of assuring compliance with these limits. The impedance provided by the resistor lowers the peak levels of the unintended radiation.
This resistor, however, has the unwanted side effect of spreading out voltage waveforms at the cathode of the cathode-ray tube, so that edges that should be sharp become blurred. This effect is particularly noticeable when a black object is displayed on a white background. If the impedance of the resistor is high enough for adequate suppression of unintended radiation, the displayed edges may be converted from sharp black-white boundaries to indistinct gray areas.
Image clarity can also be degraded by moire patterns produced by the shadow mask or aperture grille of the cathode-ray tube.